1) after the script times out, proceed from the white screen to the list of all subscribers
2) go to the very last page, copy the last email
3) open the CSV file that you are trying to import in a notepad (for example)
4) find the email
5) delete all the lines from the line with this email address up to the second line in the file (leave the first line of CSV file)
6) repeat steps 1-5 till the file is imported completely